[llvm-dev] llvm (the middle-end) is getting slower, December edition

[Mehdi Amini via llvm-dev]

> On Dec 21, 2016, at 1:57 PM, Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> 
> On 12/21/2016 03:36 PM, Sanjay Patel via llvm-dev wrote:
>> I would have replied to this thread sooner, but I was busy adding more instcombines. :)
>> 
>> The motivation for r289855 is in the commit msg (I'm out of the office and can't look things up conveniently). Feel free to revert that and the follow ups, however, if that patch caused a noticeable slowdown, then it suggests we have a bigger problem?...that's a simple matcher (no value tracking required).
> 
> I'd recommend against reverting this until we understand more about the problem.

I tend to agree, but I’d also like that we get better understanding before 4.0 branches.

> This is not a critical compile-time regression,

I didn’t see in this thread a mention about how much alone this is causing? The regression mentioned by Davide (20%) seems critical to me.

— 
Mehdi


> and we don't yet know the cause (it might just be enabling more down-stream transformations), or whether there have been any corresponding benefits to code side, performance, etc. It definitely looks like a pattern that we should catch.
> 
>  -Hal
> 
>> 
>> On Wed, Dec 21, 2016 at 12:12 PM Sean Silva <chisophugis at gmail.com <mailto:chisophugis at gmail.com>> wrote:
>> On Wed, Dec 21, 2016 at 8:07 AM, Davide Italiano via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> On Mon, Dec 19, 2016 at 4:24 PM, Mikhail Zolotukhin
>> 
>> 
>> <mzolotukhin at apple.com <mailto:mzolotukhin at apple.com>> wrote:
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>> 
>> > Hi Davide,
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>> 
>> >
>> 
>> 
>> > Thanks for the analysis, it's really interesting! And I'm really glad that we now put more and more attention at the compile time!
>> 
>> 
>> >
>> 
>> 
>> > Just recently I've been looking into historical compile time data as well, and have had similar conclusions. The regressions you've found are probably caused by:
>> 
>> 
>> > 1) r289813 and r289855 - new matchers in InstCombine
>> 
>> 
>> > 2) r286814 and r288024 - changes in Inlining cost model
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>> 
>> >
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>> 
>> 
>> 
>> 
>> Haven't looked at 2) yet, but I can confirm for 1). Sanjay/Ehsan, can
>> 
>> 
>> you please explain what's the motivation behind the new
>> 
>> 
>> transformations you introduced? I'm tempted to ask a revert, but I'd
>> 
>> 
>> like to understand the motivations first.
>> 
>> 
>> Both r289813 and r289855 add a very small amount of matching (it seems?) compared to the rest of the size of instcombine. How is it that these checks are causing such a disproportionate slowdown compared to the rest of instcombine? (by "I can confirm for 1)" I assume you mean these two patches have a pretty significant impact on compile time; not "0.1%" each)
>> 
>> -- Sean Silva
>>  
>> 
>> 
>> 
>> --
>> 
>> 
>> Davide
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>> 
>> 
>> 
>> 
>> "There are no solved problems; there are only problems that are more
>> 
>> 
>> or less solved" -- Henri Poincare
>> 
>> 
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> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
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